Glucuronidation by UGTs is a major Phase II metabolic pathway, widely distributed in the body, responsible for conjugation of numerous drugs, other xenobiotics and endogenous substrates. Glucuronidation is generally a detoxification process, though there are notable exceptions. Glucuronidation also directs the excretion and distribution of xenobiotics such that the conjugates are often substrates for active transport into the urine and excretion in bile. Biliary excretion is one step in the enterohepatic recycling (EHC) of drugs that are glucuronidated and it has the effects of reducing the apparent clearance and increasing intestinal exposure to drugs. Biliary excretion has been implicated as enhancing intestinal toxicity by cytotoxic drugs such as the antitumor agent irinotecan. Previous approaches to the problem of intestinal toxicity, with the integration of drug metabolism, biliary excretion and EHC have had some success, though predictive relationships or approaches to reduce intestinal toxicity are often still elusive. A critical component of drug-induced intestinal toxicity that has heretofore not been incorporated into the scheme, is the ability of intestinal epithelial cells to form glucuronides and excrete them, thus providing intrinsic resistance to toxicity at the cellular level. Mycophenolic acid (MPA, immunosuppressant) and SN-38 (antitumor), are active compounds derived from prodrugs MMF and irinotecan, respectively, that rely upon glucuronidation and biliary excretion for elimination. Both are cytotoxic drugs that have a high incidence of intestinal toxicity manifested by diarrhea that often limits effective therapy. Intestinal UGT is under active investigation to characterize its potential role in protection from exogenous toxins and methods to identify and measure expression of specific human isoforms of UGT are developing rapidly from the efforts of numerous labs. Large interpatient variability and potential polymorphisms of UGTs in humans are also not fully understood. Some labs have identified tissue distributions of UGT isoforms based upon qualitative RT-PCR, however, these measures do not provide information on the quantitative expression and functional capacity of these enzymes. Here we propose to investigate the intestinal toxicity of MPA and SN38, in humans and animals, by integrating toxicokinetics with protein expression of intestinal UGTs and their direct catalysis of substrates to form glucuronides. The following global hypothesis will be addressed: Intestinal UGTs have a critical role in modulating exposure of intestinal epithelial cells to drugs, thus influencing their of gi toxicity. A series of in vitro studies with cell culture and human tissues, and in vivo studies in Gunn and TR- rat models will be conducted using specific assays of MPA and SN-38 glucuronidation together with specific Western blots of rat and human UGT isozymes to determine the relationship between glucuronidation and toxicity. Translational studies to patients receiving these drugs will also be conducted to evaluate intestinal glucuronidation in humans and examine the hypothesis that local tissue specific UGTs may modulate intestinal toxicity in humans.